Glass-based gradient refractive index (GRIN) materials, often called in the literature GRIN materials, have been studied for several years. Only in recent years, however, has this technology been brought to the market place with regard to large (e.g., at least 5 mm along the optical path), macrogradient (e.g., .DELTA.n greater than 0.085) lens blanks, under the trademark GRADIUM.RTM., by LightPath Technologies (Albuquerque, N.Mex.).
Glass-based gradient refractive index materials find a wide variety of uses, including imaging lenses in optical systems, such as still and video cameras, copying machines, binoculars, microscopes, endoscopes, etc., as non-imaging lenses in concentrators, such as solar concentrators, collimators, such as laser collimators, and wavelength division multiplexing/demultiplexing applications.
The technology of the glass-based GRIN has been significantly advanced by LightPath Technologies, who developed a fusion/diffusion process to fabricate macro-size glass axial gradient lens blanks with controllable profiles of the refractive index gradient and with large .DELTA.n (the difference in refractive index from one surface to the opposite surface of the lens).
Plastic optical materials have been developed that compete with glass in several markets, due to the advancements in technology that drives down costs, speeds up production, provides consistency, improves relative quality, and allows the manufacturer to produce more such plastic lenses in less time, and saves money in the process. The final products are in general significantly lighter and cheaper. Because of the relatively very low process temperatures, compared to glass, the polymer lenses offer considerable economies of scale in production.
One advantage that plastic lenses have over glass lenses is their light weight. The specific gravity of a typical, high quality polymer, e.g., polycarbonate, is 1.2 g/cm.sup.3, whereas the specific gravity of the glass counterpart could be three to five times higher. Certain applications where such light weight would be an obvious advantage include space applications using solar photovoltaic cells with a concentrator for converting solar radiation into electricity. The electric power-to-weight ratio, i.e., watts generated per kilogram added is a significant consideration. Another application involves night vision goggles, which comprise complex infrared optics mostly made of glass, which contributes substantially to the overall weight of the headgear to which they are attached. In certain emergencies where a pilot wearing such a headgear has no time to remove it, the generated inertial forces can cause severe neck injuries to the pilot. Finally, yet another application is prescription eye glasses, where the weight is highly undesirable, because heavy glasses are uncomfortable, tend to slide down, leave heavy marks on the skin, and dislodge more quickly in the case of an abrupt acceleration or deceleration.
A significant body of literature has developed, reporting on various polymer gradient refractive index (P-GRIN) materials, including applications in (a) optical fibers for fiber networks for avionics, automotive, aerospace, and computers; (b) lens rods for light coupling, collimating, and imaging; (c) waveguides for opto-electronic integrated circuits; (d) microlenses for light coupling into single mode optical fibers; (e) macrolenses for prescription and contact lenses; and (f) optical fiber amplifiers for optical fiber lasers.
Many of the polymer gradient refractive index materials are fabricated by one of a number of reported techniques, including chemical copolymerization, gas or vapor phase diffusion copolymerization, photo-copolymerization, modified suspension polymerization, sulfonation, deposition by plasma-enhanced chemical vapor deposition, use of curved molds, interfacial gel polymerization, initiator diffusion method, and random copolymerization, for example.
However, these techniques suffer from one or more of the following disadvantages: (1) only small size optical elements, on the order of millimeters, can be produced; (2) a long time to fabricate optical elements is required; (3) only low .DELTA.n values, on the order of 0.02, are possible; and (4) no accurate prescribable tailoring of gradient profile is possible, thereby making available only specific profiles, as in the case of liquid-phase and vapor-phase diffusion techniques for thick and large samples; see, e.g., Yasukiro Koike et al, "Plastic axial gradient-index lens", Applied Optics, Vol. 24 (24), pp. 4821-4325 (Dec. 15, 1985).
Thus, what is needed is a process for fabrication of polymer gradient refractive index lenses that avoids most, if not all, of the foregoing problems.